So far, ultrahigh pressure water jet equipment and sand blasting equipment have been put to practical use as surface treating devices which eject a high pressure fluid such as water, or a high pressure fluid mixed with abrasive grains, toward the surface of an object, such as an oil reservoir tank or a ship hull, thereby removing foreign matter, such as an old coating or rust, stuck onto the surface of the object; or roughening the surface of the object with the abrasive grains ejected, to make the surface prepared for coating; or cleaning the surface of the object to carry out non-destructive testing.
The devices disclosed in the specification and drawings of U.S. Pat. Nos. 4,095,378 and 4,934,475 are typical examples of a surface treating device which ejects a high pressure fluid such as water, or a high pressure fluid mixed with abrasive grains, toward the surface of an object while suction-adhering to the surface of the object by the pressure of an ambient fluid such as air or water and moving along the surface, thereby removing foreign matter, such as an old coating or rust, stuck onto the surface of the object; or roughening the surface of the object with the abrasive grains ejected, to make the surface prepared for coating; or cleaning the surface of the object to carry out non-destructive testing.
Such a device capable of suction-adhering to the surface of an object and moving along it has a case, a surface sealing means mounted on the case and defining a pressure reduction space in cooperation with the case and the surface, and a pressure reducing means for discharging a fluid from the pressure reduction space to decrease the pressure inside the pressure reduction space. The device can be moved by wheels as moving means, other suitable element such as an endless track, or other suitable moving means such as a vibration generating means. Upon actuation of the pressure reducing means, fluid inside the pressure reduction space is discharged to the outside. Owing to the difference in fluid pressure between the inside and outside of the pressure reduction space, the fluid pressure acts on the case, and is then transmitted to the surface of an object via the wheels or other receiver member. Under this fluid pressure, the device is caused to suction-adhere to the surface of the object. By driving the moving means in this state of suction-adhesion, the device moves along the surface of the object. On this device is mounted working unit such as a surface treating material ejecting means for ejecting a surface treating material, such as high pressure water or abrasive grains, toward the surface of the object. In the present specification, the surface treating material refers to a material, such as a high pressure fluid including high pressure water or compressed air, abrasive grains, or the high pressure fluid mixed with abrasive grains, which can treat the surface of an object when ejected from a nozzle to the surface.
In conventional ultrahigh pressure water jet equipment, about 20 liters/minute of water pressurized to an ultrahigh pressure of about 2,000 to 2,500 kilograms/cm.sup.2 is ejected from a nozzle with a diameter of 0.1 to 0.5 mm. Furthermore, this nozzle is revolved at a high speed of about 1,000 rpm with a diameter of gyration of about 400 mm on a plane parallel to the surface of an object 20 to 30 mm apart from the surface. During revolution, the nozzle is gradually moved along the surface of the object at a rate of about 3 meters/minute. Thus, the surface of the object is treated continuously. The reason why the nozzle is revolved at a high speed is that the diameter of the nozzle is so small that its width of treatment is also small. If the nozzle is revolved at a low speed, the problem arises that the surface after treatment has been treated in a linear form resulting only from a trail of the center of the moving nozzle, and not in a surface form. The connection between an ultrahigh pressure hose for supplying the nozzle with ultrahigh pressure water and the nozzle is by a swivel joint. That is, the use of a swivel joint has thus far been absolutely necessary to supply ultrahigh pressure water to the nozzle revolved at a high speed. This type of joint becomes out of order relatively frequently. This use of a swivel joint is likely to make the performance of the ultrahigh pressure water jet equipment unstable.
A possible method of mixing abrasive grains into a stream of ultrahigh pressure water to be ejected from the nozzle is to mix abrasive grains into an ultrahigh pressure water stream on the upstream side of the nozzle. However, the nozzle for feeding the ultrahigh pressure water in the ultrahigh pressure water jet equipment has a small diameter, thus making it difficult to mix abrasive grains, larger in diameter than the nozzle, into an ultrahigh pressure water stream on the upstream side of the nozzle. The mixing of abrasive grains into an ultrahigh pressure water stream therefore requires that a mixing nozzle for mixing ultrahigh pressure water with abrasive grains be provided downstream of the nozzle. Such a method is already in practical use with ultrahigh pressure water jet equipment of a type in which the nozzle is not revolved (e.g. ultrahigh pressure water jet equipment applied to cutting).
In the aforementioned ultrahigh pressure water jet equipment, however, the nozzle is revolved at a high speed, thus requiring that a swivel joint be used for connection between the nozzle and the ultrahigh pressure hose for supplying ultrahigh pressure water to the nozzle. In order to mix abrasive grains into a stream of ultrahigh pressure water, therefore, ultrahigh pressure water and abrasive grains have to be supplied separately to the rotating nozzle through two hoses, i.e., a hose for ultrahigh pressure water and a hose for abrasive grains, and consequently, a swivel joint for a two-phase flow is required. However, such a swivel joint is difficult to produce. Thus, the aforementioned ultrahigh pressure water jet equipment poses difficulty in mixing abrasive grains into an ultrahigh pressure water stream to be ejected from the nozzle revolved at a high speed. Its treating capacity has clear limitations. Specifically, because of the difficulty in mixing abrasive grains into an ultrahigh pressure water stream ejected from the nozzle revolved at a high speed, it is easy to peel off a soft coating, but it is difficult to remove a hard rust. Nor can the ultrahigh pressure water jet equipment form an anchor pattern, i.e. roughness, on the surface of an object, because its formation absolutely requires that abrasive grains be ejected toward the surface of the object and caused to impinge thereon.
In the sand blasting equipment which roughens the surface of an object by directing a jet of abrasive grains at the surface of the object, the use of a swivel joint is absolutely necessary to supply abrasive grains to the nozzle revolved at a high speed. This use of a swivel joint is likely to make the performance of the equipment unstable.
Also in the surface treating device capable of suction-adhering to the surface of an object by the pressure of an ambient fluid such as air or water and moving along the surface, the use of the nozzle revolved at a high speed requires the use of a swivel joint as a means of feeding a high pressure water stream or abrasive grains to the nozzle. Owing to this use of a swivel joint, the performance of the equipment may become unstable. For the same reasons as stated previously, moreover, it is difficult to mix abrasive grains into a high pressure water stream ejected from the nozzle revolving at a high speed. Thus, this device also faces the same problems as does the ultrahigh pressure water jet equipment.